1. Field of the Invention
The invention relates to and has among its objects the inhibition of lysinoalanine formation which occurs when treating proteinaceous materials with alkali. Further objects of the invention will be evident from the following description wherein parts and percentages are by weight unless otherwise specified.
2. Description of the Prior Art
Proteinaceous materials are commonly treated with alkali in the course of preparing protein concentrates or isolates for food or feed use. For example, in preparing soy protein concentrates a usual step is to extract soybeans with aqueous alkali and then precipitate the protein from the resulting solution. Similar alkali treatments are used in recovering proteins from such sources as cereal grains and milling by-products, and oilseeds such as peanuts, safflower seed, cottonseed, flaxseed, etc. Procedures are also known wherein proteins--such as soy protein--are modified by treatment with alkali to induce optimum adhesive properties for use in preparing textured foods.
Heretofore, it has been shown that when rats are fed a diet containing alkali-treated soy protein they suffer nephrotoxic effects. Woodward et al (Journal of Nutrition, Vol. 103, pages 569-574) investigated this problem and found that the nephrotoxic factor in the alkali-treated protein is heat stable and cannot be extracted with such solvents as water, methanol, chloroform, and hexane. Their studies demonstrated that the toxic factor is an unusual amino acid which is chemically bound to the protein chain, and that this amino acid is N.sup.68 -(DL-2-amino-2-carboxyethyl)-L-lysine. They assigned to it the abbreviated name "lysinoalanine," the latter being also used herein.
Lysinoalanine has the structure - ##STR1##
It is evident from the foregoing that the presence of lysinoalanine in products designed for use by humans or animals is undesirable.
Wool and other keratinous fibers are subjected to hot, alkaline conditions when treated with detergents, etc. Under such conditions lysinoalanine formation occurs resulting in a deterioration of many of the desired properties of the wool, e.g., hand, tensile strength, abrasion resistance, etc. Furthermore, lanthionine is also produced under alkaline conditions and this substance further contributes to the aforementioned deterioration in the desired properties of the wool.
In U.S. Pat. No. 4,035,349, there is described an improvement in the process of treating vegetable protein with alkali. In the patented method lysinoalanine formation is inhibited during alkaline treatment of protein by conducting the treatment in the presence of an added compound selected from the group consisting of sulfhydryl-containing amino acids, their esters, and amides in an amount sufficient to prevent lysinoalanine formation.
It is believed that when proteinaceous material is treated with alkali, lysinoalanine is produced in the following way: The alkali acts upon the cystine portion of the protein and degrades it to a dehydroalanine unit, which subsequently reacts with the .epsilon.-amino group of a lysine unit to form a lysinoalanine unit in the protein molecule. These reactions may be represented schematically as follows: ##STR2##
The sequence of reactions culminating in the formation of lanthionine may be illustrated as follows: ##STR3## Cysteine in the presence of alkali combines with a dehydroalanine unit to give lanthionine.
One way to prevent lysinoalanine formation would be to block the .epsilon.-amino group of lysine, such as by acylation, and thus render it incapable of reacting with dehydroalanine. This approach, however, would not be considered by the art skilled person for the following reasons. The resultant amide that formed would not be expected to be stable to the alkaline conditions employed in the treatment of the proteinaceous material. It would be anticipated that the amide would be hydrolyzed in the presence of alkali, thereby regenerating the .epsilon.-amino group of lysine, which would then react with dehydroalanine to produce lysinoalanine. Furthermore, even if it were assumed that the blocking group would be stable under alkaline conditions, one would expect the resultant protein product to be deficient in lysine, which would not be considered to be nutritionally available from its acylated (amide) derivative.
A final note is that blocking the .epsilon.-amino group of lysine would not be expected to have any effect whatsoever on the amount of lanthionine produced in the alkaline treatment of keratinous fibers. Lanthionine is formed by bonding between the vinyl group of the dehydroalanine unit and the sulfhydryl group of cysteine; acylation of the above-mentioned nature should not prevent such bonding.